Semiconductor device and fabricating method thereof

ABSTRACT

A semiconductor device is provided. The device includes a semiconductor substrate and a gate structure thereon. A well region is formed in the semiconductor substrate. A drain region and a source region are respectively formed in the semiconductor substrate inside and outside of the well region. At least one set of the first and second heavily doped regions is formed in the well region between the drain region and the source region, wherein the first and second heavily doped regions are stacked vertically from bottom to top and have a doping concentration which is larger than that of the well region. The semiconductor substrate and the first heavily doped region have a first conductivity type and the well region and the second heavily doped region have a second conductivity type. A method for fabricating a semiconductor device is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, and inparticular, to a semiconductor device having a super junction structureand a method for manufacturing the same.

2. Description of the Related Art

Semiconductor devices, such as high voltage elements, are typicallydivided into: vertical double-diffused metal-oxide-semiconductor fieldeffect transistors (VDMOSFETs) and laterally diffusedmetal-oxide-semiconductor field effect transistors (LDMOSFETs). In orderto increase the withstand voltage of the described high voltageelements, the doping concentration of the deep well region (or referredto as the drift region) is reduced, the depth of the drift region isincreased, or the length of the isolation structure (or referred to asthe field oxide layer) underlying the gate is increased.

FIG. 1 is a cross section of a conventional n-type LDMOSFET. The n-typeLDMOSFET 10 comprises a p-type semiconductor substrate 100 and a p-typeepitaxial layer 102 thereon. A gate structure 116 and a field oxidelayer 114 are on the p-type epitaxial layer 102. Moreover, a p-type bodyregion 106 and an n-type drift region 104 are respectively in the p-typeepitaxial layer 102 on both sides of the gate structure 116, wherein then-type drift region 104 further extends into the underlying p-typesemiconductor substrate 100. A p-type contact region 108 and an adjacentn-type contact region 110 (or both referred to as a source region) arein the body region 106 and an n-type contact region 112 (or referred toas a drain region) is in the drift region 104. Moreover, a sourceelectrode 117 is electrically connected to the p-type contact region 108and the n-type contact region 110. A drain electrode 119 is electricallyconnected to the n-type contact region 112. A gate electrode 121 iselectrically connected to the gate structure 116.

As mentioned above, in order to improve the withstand voltage of thetransistor 10, the doping concentration of the drift region 104 has tobe reduced and/or the length of the field oxide layer 114 underlying thegate structure 116 has to be increased. However, when the withstandvoltage is increased by the described ways, the on-resistance (Ron) orthe size of the transistor 10 is also increased.

Thus, there exists a need in the art for development of a semiconductordevice, capable of increasing the withstand voltage while preventing theon-resistance from increasing.

BRIEF SUMMARY OF INVENTION

A detailed description is given in the following embodiments withreference to the accompanying drawings. Semiconductor devices andmethods for fabricating the same are provided.

An exemplary embodiment of a semiconductor device includes asemiconductor substrate of a first conductivity type. A well region of asecond conductivity type is formed in the semiconductor substrate. Adrain region and a source region are respectively formed in thesemiconductor substrate inside and outside of the well region. At leastone set of the first and second heavily doped regions is formed in thewell region between the drain region and the source region, wherein thefirst and second heavily doped regions of the first and secondconductivity types, respectively, are stacked vertically from bottom totop and have a doping concentration which is larger than that of thewell region. A gate structure is disposed on the semiconductorsubstrate.

An exemplary embodiment of a method for fabricating a semiconductordevice comprises providing a semiconductor substrate of a firstconductivity type. A well region of a second conductivity type is formedin the semiconductor substrate. At least one set of the first and secondheavily doped regions is formed in the well region, wherein the firstand second heavily doped regions of the first and second conductivitytypes, respectively, are stacked vertically from bottom to top and havea doping concentration which is larger than that of the well region. Adrain region and a source region are respectively formed in thesemiconductor substrate inside and outside of the well region, such thatthe set of first and second heavily doped regions is in the well regionbetween the drain region and the source region. A gate structure isformed on the semiconductor substrate.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a cross section of a conventional n-type LDMOSFET.

FIGS. 2A to 2D are cross sections of an exemplary embodiment of a methodfor fabricating a semiconductor device according to the invention.

DETAILED DESCRIPTION OF INVENTION

The following description is of a mode for carrying out the invention.This description is made for the purpose of illustrating the generalprinciples of the invention and should not be taken in a limiting sense.The scope of the invention is best determined by reference to theappended claims.

FIG. 2D illustrates a cross section of an exemplary embodiment of asemiconductor device 20 according to the invention. In the embodiment,the semiconductor device 20 may be a laterally diffusedmetal-oxide-semiconductor field effect transistor (LDMOSFET) havingsuper junction structures. Moreover, the semiconductor device 20comprises a semiconductor substrate 200, such as, a silicon substrate,silicon on insulator (SOI) substrate or other suitable semiconductorsubstrate, of a first conductivity type.

A well region 204, a source region 218, a drain region 220 and a bodyregion 212 are formed in the semiconductor substrate 200. For example,the well region 204 of a second conductivity type opposite to the firstconductivity type extends into the semiconductor substrate 200 from thetop surface of the semiconductor substrate 200. Moreover, the wellregion 204 corresponds to an active region A (which is defined by aportion of the isolation structure, such as the field oxide layer 214)of the semiconductor substrate 200 to serve as a drift region of theLDMOSFET.

The source region 218 comprises a doped region 218 a of the secondconductivity type and a doped region 218 b of the first conductivitytype. The source region 218 is formed in the semiconductor substrate 200outside of the well region 204 and corresponds to the active region A.Moreover, the body region 212 of the first conductivity type is formedin the semiconductor substrate 200 outside of the well region 204, suchthat the source region 218 is in the body region 212. The drain region220 is formed of the doped region of the second conductivity type only.The drain region 220 is formed in the well region 204 and corresponds tothe active region A.

At least one set of the first heavily doped region 201 and secondheavily doped region 203 is formed in the well region 204 between thedrain region 220 and the source region 218, wherein the first heavilydoped region 201 and the second heavily doped region 203 are stackedvertically from bottom to top and the first heavily doped region 201 iselectrically floating. The first and second heavily doped regions 201and 203 are respectively of the first and second conductivity types andhave a doping concentration which is larger than that of the well region204 to form a super junction structure in the well region 204 of thesemiconductor substrate 200. In the embodiment, the first conductivitytype is p-type and the second conductivity type is n-type.Alternatively, the first conductivity type is n-type and the secondconductivity type is p-type.

In other embodiments, the semiconductor device 20 may comprise aplurality of sets of the first and second heavily doped regions 201 and203 that is stacked vertically in the well region 204 of thesemiconductor substrate 200 to form a plurality of the super junctionstructures in the semiconductor substrate 200.

A gate structure 216 is disposed on the semiconductor substrate 200 andbetween the source region 218 and the drain region 220. The gatestructure 216 typically comprises a gate (e.g., a polysilicon gate), agate dielectric layer underlying the gate and a field oxide layer 214underlying the gate dielectric layer.

The semiconductor device 20 further comprises an interlayer dielectric(ILD) layer 226 and a plurality of interconnect structures 221,223 and225 therein. In the embodiment, the interconnect structure 221 iselectrically connected to the source region 218 to serve as a sourceelectrode, the interconnect structure 223 is electrically connected tothe gate structure 216 to serve as a gate electrode and the interconnectstructure 225 is electrically connected to the drain region 220 to serveas a drain electrode.

In the foregoing embodiment, the heavily doped region of the firstconductivity type and electrically floating in the super junctionstructure may help in the formation of a depletion region in the wellregion 204 (i.e., the drift region), thereby improving the withstandvoltage of the LDMOSFET in the semiconductor device 20. Moreover, theheavily doped region of the second conductivity type in the superjunction structure may provide an additional current path in the wellregion 204 (i.e., the drift region) to reduce the on-resistance betweenthe source region and the drain region.

FIGS. 2A to 2D are cross sections of an exemplary embodiment of a methodfor fabricating a semiconductor device 20 according to the invention.Referring to FIG. 2A, a semiconductor substrate 200, such as a siliconsubstrate, a silicon on insulator (SOI) substrate or other suitablesemiconductor substrates, of a first conductivity type is provided.Next, a well region 204 is formed in a predetermined region (i.e., anactive region A) of the semiconductor substrate 200 sequentially by adoping process (e.g., ion implantation), and a thermal diffusionprocess, wherein the well region 204 of a second conductivity typedifferent from the first conductivity type is configured to serve as adrift region of a subsequently formed LDMOSFET.

In the embodiment, one set of the first and second heavily doped regions201 and 203 may be formed in the well region 204, wherein the first andsecond heavily doped regions 201 and 203 are stacked vertically frombottom to top. The first and second heavily doped regions 201 and 203are respectively of the first and second conductivity types and have adoping concentration which is larger than that of the well region 204 toform a super junction structure in the well region 204 corresponding tothe semiconductor substrate 200.

In other embodiments, a plurality of sets of the first and secondheavily doped regions 201 and 203 are formed in the well region 204 ofthe semiconductor substrate 200. The plurality of sets of the first andsecond heavily doped regions 201 and 203 substantially and verticallyalign to each other to form a plurality of super junction structures inthe well region 204 of the semiconductor substrate 200.

In the foregoing embodiment, the first and second heavily doped regions201 and 203 are between a drain region 220 and a source region 218 (asshown in FIG. 2C) that are subsequently formed, wherein the firstheavily doped region 201 is electrically floating. In the embodiment,the first conductivity type is p-type and the second conductivity typeis n-type. Alternatively, the first conductivity type is n-type and thesecond conductivity type is p-type.

Referring to FIGS. 2B and 2C, a plurality of isolation structures, suchas a field oxide layer 214, may be formed on the semiconductor substrate200 by the conventional MOS process, wherein the active region A isdefined by a portion of the field oxide layer 214 and a drain region Dto be formed in the well region 204 is defined by the rest of the fieldoxide layer 214. Afterwards, a gate structure 216 is formed on thesemiconductor substrate 200 to define a source region S to be formed inthe active region A outside of the well region 204, as shown in FIG. 2B.

Next, a body region 212 of the first conductivity type is optionallyformed in the semiconductor substrate 200 outside of the well region 204sequentially by a doping process (e.g., ion implantation), and a thermaldiffusion process, such that the subsequently formed source region 218is in the body region 212. A doped region 218 a of the secondconductivity type is then formed in the source region S to be formed (asshown in FIG. 2B) and another doped region (i.e., the drain region 220)of the second conductivity type is formed in the drain region D to beformed (as shown in FIG. 2B) by a doping process (e.g., ionimplantation). Afterwards, a doped region 218 b of the firstconductivity type is formed in the source region S to be formed (asshown in FIG. 2B) and adjacent to the doped region 218 a, such that thedoped region 218 b and the doped region 218 a form a source region 218,as shown in FIG. 2C.

In other embodiments, the doped region 218 b may be formed beforeforming the doped region 218 a and the drain region 220. In theembodiment, the doped region 218, the gate structure 216, the drainregion 220 and the well region 204 having the super junction structuresform an LDMOSFET.

Referring to FIG. 2D, an interlayer dielectric layer 226 and a pluralityof interconnect structures 221, 223 and 225 therein are formed on thesemiconductor substrate 200 by the conventional metallization process.The interconnect structure 221 is electrically connected to the sourceregion 218 to serve as a source electrode, the interconnect structure223 is electrically connected to the gate structure 216 to serve as agate electrode and the interconnect structure 225 is electricallyconnected to the drain region 220 to serve as a drain electrode. As aresult, the fabrication of the semiconductor device 20 is completed.

According to the foregoing embodiments, the heavily doped region of thefirst conductivity type and electrically floating in the super junctionstructure may form a depletion region in the drift region, so that thewithstand voltage of the LDMOSFET in the semiconductor device isimproved. Moreover, the heavily doped region of the second conductivitytype in the super junction structure may provide an additional currentpath in the drift region, so that the on-resistance of the LDMOSFET isreduced. Additionally, according to the foregoing embodiments, thenumber of super junction structures stacked vertically in the driftregion may be controlled to further improve the withstand voltage of theLDMOSFET while preventing the on-resistance of the LDMOSFET fromincreasing.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate of a first conductivity type; a well region of asecond conductivity type formed in the semiconductor substrate; a drainregion and a source region respectively formed in the semiconductorsubstrate inside and outside of the well region; at least one set offirst and second heavily doped regions of the first and secondconductivity types, respectively, formed in the well region between thedrain region and the source region, wherein the first and second heavilydoped regions of the first and second conductivity types, respectively,are stacked vertically from bottom to top and have a dopingconcentration which is larger than that of the well region; and a gatestructure disposed on the semiconductor substrate.
 2. The semiconductordevice of claim 1, wherein the first heavily doped region iselectrically floating.
 3. The semiconductor device of claim 1, furthercomprising a plurality of sets of first and second heavily doped regionsstacked vertically in the well region.
 4. The semiconductor device ofclaim 1, wherein the first conductivity type is p-type and the secondconductivity type is n-type.
 5. The semiconductor device of claim 1,wherein the first conductivity type is n-type and the secondconductivity type is p-type.
 6. The semiconductor device of claim 1,further comprising a body region of the first conductivity type formedin the semiconductor substrate outside of the well region, such that thesource region is in the body region.
 7. A method for fabricating asemiconductor device, comprising: providing a semiconductor substrate ofa first conductivity type; forming a well region of a secondconductivity type in the semiconductor substrate; forming at least oneset of first and second heavily doped regions of the first and secondconductivity types, respectively, in the well region, wherein the firstand second heavily doped regions of the first and second conductivitytypes, respectively, are stacked vertically from bottom to top and havea doping concentration which is larger than that of the well region;forming a drain region and a source region respectively in thesemiconductor substrate inside and outside of the well region, such thatthe set of first and second heavily doped regions is in the well regionbetween the drain region and the source region; and forming a gatestructure on the semiconductor substrate.
 8. The method of claim 7,wherein the first heavily doped region is electrically floating.
 9. Themethod of claim 7, further comprising forming a plurality of sets offirst and second heavily doped regions in the well region, wherein theplurality of sets of first and second heavily doped regions are stackedvertically.
 10. The method of claim 7, wherein the first conductivitytype is p-type and the second conductivity type is n-type.
 11. Themethod of claim 7, wherein the first conductivity type is n-type and thesecond conductivity type is p-type.
 12. The method of claim 7, furthercomprising forming a body region of the first conductivity type in thesemiconductor substrate outside of the well region, such that the sourceregion is in the body region.